Portable electronic devices, such as electronic book (“e-book”) readers, smartphones, or tablet computers, have become increasingly popular. One constraint on usability for portable electronic devices is their limited battery life, due to smaller battery sizes, which requires users recharge the device after some period of usage. For all battery-powered electronic devices and e-book readers in particular, it is desirable to provide extended battery life to avoid forcing the user to recharge the e-book reader too frequently. Many users of e-book readers expect to go many days, weeks, or months on a single charge. Increasing the capacity of the battery to improve battery life generally requires an increase in the size and weight of the battery, and, therefore, the device as well. However, reduced weight and thickness are also highly desirable features for portable electronic devices, particularly e-book readers, which in typical usage, may be held in the user's hands for hours at a time.
In contrast with analog devices, which typically draw a steady current, digital devices place new demands on batteries, including loading the battery with periodic spikes of high current draw. These current spikes cause corresponding voltage drops, due to battery DC resistance values. Many of these digital devices are designed with cut-off voltages, such as 3.0V, at which point the device stops operating or shuts itself down. As a result, during normal operation, a temporary current spike can cause the device to display a low battery warning to the user and even shut itself down. Although the device could be restarted and operated at a low current draw for an additional period of time, the low battery warning caused by the current spike would cause the user to conclude that the battery is depleted and the device unusable until recharged again. In addition, precise measurement of remaining battery capacity is technically difficult, thereby making it particularly challenging to develop processes for maximizing battery life.
Accordingly, there is a need for improved battery systems that can minimize device weight and thickness while powering electronic devices, particularly those with high power and peak power demands.